This invention relates generally to fuel supply systems for internal combustion engines of the type that are capable of operating on either liquid fuel oil or natural gas. More specifically, it relates to a fuel injection system that is capable of selectively delivering precisely controlled quantities of fuel, varying between a minimum and a maximum that may be one hundred (100) times greater than the minimum, or more. The relationship between the maximum and minimum quantities of fuel within the range of such a fuel system is generally identified as the "turn-down ratio".
Combustion engines capable of operating, selectively, using either liquid fuel oil or natural gas, are generally well known. It is also known that when such engines are operated using natural gas as the primary fuel, it is necessary to supply the engine with minimum quantities of liquid fuel oil in addition to the natural gas. The fuel oil injected into the engine under these circumstances is generally identified as "pilot fuel". In this context, compression and consequent combustion of the pilot fuel acts as an ignition mechanism for the natural gas, to sustain operation of the engine without an electrically powered ignition system; this is the primary function of pilot fuel injection.
Engines of this type commonly produce undesirable by-products of fuel oil combustion in the form of oxides of nitrogen. The various oxides that are produced, including Nitrous Oxide and Nitric Oxide, have come to be identified by the all-inclusive coined symbol, NOx.
It has been generally known, for ten years or more, that an effective way to reduce the quantities of NOx produced by a dual-fuel engine when it is operating in the natural gas mode, is to reduce the quantities of pilot fuel supplied to the engine. The quantities of pilot fuel used by a dual-fuel engine may be expressed conveniently as a percentage of the fuel oil consumed by the engine when it is operating in the full diesel mode, at one hundred percent (100%) of its rated load. In the past, pilot quantities commonly averaged about five percent (5%) of full diesel mode fuel consumption. It has been determined that dual-fuel engines can be operated successfully using pilot fuel quantities that are as low as one percent (1%), or less, of the full diesel/full load fuel consumption, provided that the reduced fuel quantities are delivered to the engine consistently, accurately and reliably. Prior art fuel injection systems for these applications generally were not capable of meeting these requirements for supply of fuel quantities that were less than about five percent (5%) of full load consumption.
Although the relationship between reduction in pilot fuel quantities and corresponding reductions in NOx output has been known for many years, interest in exploitation of this knowledge has been limited. In general, the limitations have been a result of restrictions imposed by the economics and existing technology of available fuel oil supply systems for dual-fuel engines. Specifically, the pumps or pumping devices used in diesel fuel systems are dominated by positive/variable-displacement piston pumps of the type known as a "jerk-pump", which is characterized by a rack-adjustment mechanism. The "rack" mechanism varies the quantity of fuel delivered by the pump, by varying the length of the portion of each piston stroke during which pumping takes place. Despite many years of existence, evolution and improvements in design, rack adjustment pumps generally are not capable of delivering, reliably, minimum fuel quantities that are less than approximately five percent (5%) of the rated maximum of the pump. For this reason, dual-fuel engines in the past customarily have been operated using no less than approximately "five percent (5%) pilot fuel".
As mentioned previously, the relationship between the maximum quantity and the minimum fuel quantity that can be delivered reliably by a given pump is referred to as the turn-down ratio. It can be recognized, accordingly, that a conventional pump that is capable of delivering, reliably, minimum quantities that are not substantially less than five percent (5%) of the maximum quantity, has a turn-down ratio of twenty to one (20:1). By significant contrast, a pump, or fuel-supply system, capable of delivering precisely controlled minimum quantities of pilot fuel that represent one percent (1%) (or less) of the maximum capacity of the pump, can be seen to represent a turn-down ratio of one hundred to one (100:1). It is highly significant that the turn-down ratio of such a system is five times greater than the turn-down ratio capability of pumps and injection systems that are considered to be the best available in the prior art.
The alternative of providing a dual-fuel engine with two independent fuel injection systems, one for injecting pilot fuel quantities, and another for injecting full-diesel fuel quantities, has been considered in the past. However, this approach generally has been rejected on the basis of the excessive costs of original equipment as well as the substantial increase in prospective maintenance.
Accordingly, it is an object of this invention to provide a fuel supply system, for dual-fuel engines, that is capable of delivering reliably, pilot fuel quantities that are equal to one percent (1%) or less of the maximum fuel pumping capacity of the pump, using a common supply pump.
Another object of the present invention is to provide a unified fuel injection system for use with dual-fuel engines, that is capable of delivering pilot fuel quantities, reliably, that represent a turndown ratio on the order of one hundred to one (100:1).